The present invention relates to method for producing a diaphragm for acoustic appliances, and more particularly relates to improvements in acoustic and mechanical characteristics of a diaphragm made of beryllium having high degree of treble range reproduction.
Various methods have been developed in production of a diaphragm made of beryllium. According to "New materials, new metals and modern production processes" by Ryohei Tanaka (page 612), powder metallurgy processes and vacuum deposition processes have been typically employed in production of diaphragms made of beryllium. A different process was also proposed by the inventors of the present invention in Japanese Patent Application Sho. 63-274295 filed in 1988.
In production of a beryllium diaphragm by means of powder metallurgy process, beryllium flakes are melted in a vacuum environment to obtain a beryllium ingot which is then screened after chipping and crushing to obtain beryllium powder. Next, the powder is subjected to a hot vacuum pressing process and a hot pressed block so prepared is subjected to a hot cross rolling process to obtain a rolled plate of 20 to 100 .mu.m thickness. The rolled plate is finally subjected to a hot pressing process to obtain a diaphragm of a desired configuration.
Production via powder metallurgy is inevitably accompanied with increased inclusion of impurities, in particular BeO, in the beryllium plate. Increased presence of such impurities significantly lowers ductility of the beryllium plate which is thus made quite unsuited for the following pressing. In addition, increased inclusion of such impurities lower the sound speeds of sounds generated by a resultant diaphragm, thereby impairing the acoustic characteristics of the product. This process is also entails high cost and much labor due to its cumbersome process steps.
In the case of production via a vacuum deposition process, a beryllium ingot is prepared first to form beryllium flakes as in the case of the metallurgy process and a copper or titanium foil is used for the substrate. A beryllium layer is formed on the surface of the substrate by means of a vacuum deposition process and a beryllium plate is obtained by etching the substrate. In order to made the internal structure of this beryllium plate denser, heat treatment is applied to the beryllium plate.
Despite reduced inclusion of impurities, the product by this vacuum deposition process cannot exhibit appreciable mechanical endurance such as mechanical strength in generation of sounds of large amplitudes in the low frequency range. In addition, the product cannot provide sounds generated with high sound pressure.
The process of Japanese Patent Application Sho. 63-274295 is based on use of vacuum deposition process. More specifically, a beryllium plate is first prepared by means of a vacuum deposition process. The beryllium plate so obtained is next subjected to high temperature treatment such as a hot pressing process or a hot hydraulic pressing process.
In this case, use of the high temperature treatment under pressure raises density of the beryllium crystal without any substantial change in crystal structure. High density assures high sound pressure of sounds generated by the resultant diaphragm. The beryllium crystals formed by vacuum deposition stand almost perpendicular to the surface of the substrate in a column form which is maintained even after the high temperature treatment under pressure. Such a crystal structure does not sufficiently assure generation of sounds with high sound pressure.